If anything its probably the antivirus software (assuming you got software and not a hardware firewall...). what exactly did your son install, do you know? He should be able to fix it for you or at least walk you through it, or else he really shouldn't have been installing the stuff in the first place...
chances are you will just have to go into the antivirus program and allow yahoo.com or mail.yahoo or something, i'm not sure what the address exactly is for yahoo as I never get on mine...
o, and so you know. (cuz i see you made a new account) It has something to do with connecting to the site, not with the account itself. You could use someone else's computer and get to your account fine (chances are). If you at least know the type of antivirus software you have I could maybe help a little, but is there any way you could call him and have him walk you through allowing yahoo mail while on the phone with him>?